Molecular studies of the initial amplification of the oncogenic human papillomavirus and closely related nonhuman primate papillomavirus genomes
Human papillomaviruses (HPVs) are clinically important viruses as persistent infection with certain HPV types (high-risk types) might induce genetic instability and cancer development in host cells over the years. Although HPV-induced cancers are among the leading causes of cancer mortality worldwide (e.g., HPVs are almost always causative agents of cervical cancer), no effective therapeutics for ongoing HPV infection currently exist and the need for anti-HPV drugs to treat ongoing viral infections is strong.

HPVs infect mucosal or cutaneous keratinocytes and have developed life-cycle strategies tightly linked to the renewal program of epithelial tissue. Mimicking this host differentiation-dependent viral life cycle under laboratory conditions is time consuming and complicated. Lack of a robust model system has hindered studying HPV DNA replication and elucidating molecular mechanisms behind it, which is a crucial prerequisite for developing effective therapeutic anti-HPV strategies. For this reason, our research group has developed a human U2OS cell line-based assay system that has proven to be an efficient tool for studying HPV genome replication. The present study made use of the U2OS cell line-based assay system. To shed more light on HPV DNA replication mechanisms, we analyzed episomal HPV replication structures generated during the initial phase of viral DNA replication. Two distinct DNA replication mechanisms were revealed to be involved in HPV genome replication - bidirectional theta-mode replication, and a unidirectional DNA replication mode which is likely associated with recombination-dependent replication. Additionally, this research characterizes cynomolgus macaque papillomaviruses (MfPVs) as potentially valuable for the development of an in vivo animal model for preclinical testing of anti-HPV therapeutics.

The findings revealed that MfPVs and HPVs share similar characteristics in gene expression and DNA replication, including the finding that MfPV replication is sensitive to HPV-specific inhibitors. These findings justify the use of cynomolgus macaques as a relevant model system for developing therapeutic antiviral strategies.